As shown in Published Japanese Translation of PCT Application Hei 11-501152 or the pamphlet of International Patent Publication WO 05/041245, a known electrodeless discharge lamp apparatus of this kind is the apparatus with the so-called internal winding method, which comprises a light-transmitting bulb containing a discharge gas and a high frequency electromagnetic field generator (hereafter referred to as coupler) accommodated in a cavity formed in the bulb for generating a high frequency electromagnetic field. In such apparatus, the coupler comprises: an induction coil through which a current flows to generate a high frequency electromagnetic field; a core formed of a soft magnetic material and inserted into the induction coil; a heat conductor for conducting heat generated by the induction coil and/or the core to near an inlet of a cavity; and a bobbin made of resin which accommodates and holds the core and/or the heat conductor, and around which the induction coil is wound.
Such an electrodeless discharge lamp apparatus has features that it has a long life because of having no electrode, good responsiveness when lighting, and high efficiency, so that it is suitably used e.g. in lighting fixtures for high ceilings, downlights and road lamps where maintenance is difficult.
Now, in recent years, attempts have been made in such electrodeless discharge lamp apparatus to increase the output of the apparatus, and development of an apparatus which operates with a lamp power over 200 W is desired. However, if an apparatus is operated with a lamp power over 200 W, the amount of heat generation becomes very large. Thus, it is necessary to increase the heat dissipation of the coupler accommodated in the cavity. More specifically, with an increase in the output of the apparatus, there is a risk that if the heat dissipation of the coupler is poor, the core approaches saturation when excessive heat is applied to the coupler, so that the coil may become unable to maintain stable impedance, causing unstable lighting. In addition, an increase in the output of the apparatus causes an increase in temperature of the bulb, causing an increase of stress to a fluorescent material coated on an inner surface of the bulb. Thus, if the heat dissipation of the coupler is poor, there is a risk that the light emission efficiency of the lamp may be reduced. For this reason, an increase in the heat dissipation of the coupler to suppress the temperature increase of the bulb and the coupler leads to prevention of the reduction in the light emission efficiency of the apparatus.
Note that the apparatus shown in the former of the above documents is designed so that the heat conductor occupies at least half of an outer peripheral surface of the coupler so as to increase the heat dissipation of the coupler. On the other hand, the apparatus shown in the latter document is designed so that a coil is wound on the surface of a skeleton-shaped bobbin and a core, in which the core placed in an opening formed by the skeleton is substantially surface-contacted with the heat conductor so as to increase the heat dissipation of the coupler.